The Secret Empire (1979)

In this homage to the cliffhanger serials of the '30s and '40s, 19th Century Wild West Marshal Jim Donner stumbled upon a secret underground city known as Chimera while chasing a gang of robbers in Wyoming. Chimera is lorded over by an evil despot named Thorval. Donner teams up with the resistance rebels in their effort to overthrow Thorval's cruel reign, while simultaneously investigating the gold robberies on the surface. Originally aired as part of NBC's Cliffhangers TV series which featured three revolving serials including this one.